well it depends on the engine really...
stroker crank plus 60mm bore can provide over 160cc.
also you cant just upgrade the engine size and think the rest of the engine will be fine...
stock gearbox, clutch and kick start gears will die within no time...
as originally posted above...
port work, after market camshaft and valve springs add some compression and a good jap spec carby(oko or pe26mm) and youll be surprised at the increase in power...
but also youll spend all this money on the 125 and for similar money you could just buy a 155 or 140 or even a 160 and be infront..